How do I engage 4 wheel drive in my 1998 jeep

SOURCE: 4 wheel drive problem on Jeep YJ

Check the Vac, seloind on the front axel

SOURCE: 1998 Ford F-150 4-wheel drive won't engage

It sounds as if you have a vacuum solenoid problem.
The 4x4/4x2 vacuum solenoids are attached to the upper RH side of the engine compartment, and alternately route vacuum to the vacuum motor which engages/disengages the center axle disconnect collar in the front axle assembly.They are grounded when activated, open circuit when deactivated.I would first of all make sure that you have power to them on the W/LB wire with ignition in run postion.If not then check fuse 23 (10amp) in the interior fuse panel.Disconnect the solenoids and measure their resitance should be between 50-100 ohms.
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SOURCE: 1999 jeep grand cherokee all time four wheel drive will not engage

Sounds like the splines on your transfer case gear are ground down

SOURCE: 89 jeep cherokee 4 wheel drive won't engage gears

If I recall correctly, there is a small vacuum line that connects to the transfer case that sometimes clogs or pops off, preventing the 4wd from engaging.......

SOURCE: 2004 jeep liberty stuck in 4 wheel drive low -

a reluctant transfer case can sometimes be coaxed by shifting into reverse, backing up 10 feet, and then back to forward.